1. Field of the Invention
The present invention relates to the testing of the integrity of the seal securing a container lid or closure on a container body and a container structure whose seal is adapted to be tested.
2. Description of the Related Art
At the present time comestibles such as yogurt, cheese spread or other perishable items, are often packed in containers of various types, such as bowls, trays and other containers which are opened by peeling off their lids. The containers may have any shape and configuration, for example, cylindrical, rectangular, oval, oblong, etc. One type of container has a bowl-like body, a bottom wall and a flexible lid.
The lid usually is, but need not be a multi-layer sheet or laminate, preferably flexible in the form of a thin, flexible disc. The lid may consist of layers of plastic which are laminated on opposite sides of a layer of aluminum foil. The bottom layer of the lid is usually a heat-sealable or adherable plastic. The container body is filled and its lid is placed on a top surface usually the flange, of the container body. The outer peripheral portion of the lid, at its bottom face, is hermetically sealed or adhered to the flange.
If the lid-flange seal has even a tiny hole, void or interruption micro-organisms may enter the container and spoil the food or other perishable product. Such spoilage may cause consumers to reject the brand of the product in the future. The consumer may resent having the inconvenience of discarding the container or returning it to the store. In addition, the spoiled food, if eaten, may cause illness, and may also seriously damage the reputation of the manufacturer and/or food producer or packer.
Food manufacturers are aware of the danger of spoilage due to seals which are not hermetic, and seek to prevent such faults by quality inspection of their filled containers. Generally, such quality inspection in the case of high acid food products includes testing for faulty seals by pulling a few containers from the production line and visually testing them for leakage. That type of quality control, based on a statistically meaningful random sample, is well adapted to detect machine errors which cause faulty seals on all the containers in a production run. However, statistical quality control is not well adapted to detect random faulty seals, for example, a pin hole in the seal of one container out of a production run of 10,000 containers.
It is realized that it would be preferable for high acid foodstuffs and it is generally required for low acid sterilized, i.e., retorted shelf-stable foodstuffs, to test each and every container, which is called "100% testing". The ideal is to test each container twice, called "200% testing". Such testing for retortable containers would preferably be accomplished before and after the filled, sealed container and its contents are sterilized. For filling, sealing and packing operations high speed is preferred. To be compatible with such filling, sealing and packaging systems, seal integrity testing systems should run at similar high speeds.
The seal area of containers of low acid foods having a heat-sealed lid are presently being individually visually 100% inspected rather than automatically tested by machine, due to the difficulty of such testing at the requisite high speeds of production. Several leak detection systems have been in development to solve the problem of high speed automatic testing of package seal integrity in a non-destructive way. Electronic, thermal and pressure differential systems have been proposed. However, for one reason or another there are problems with each of these systems and none is totally successful in addressing all the needs of a fully commercial testing system. The system should be fast and must be sensitive, non-destructive, automatic, reliable and accurate.